The transcription states of genes are either stable or flexible, which has clear biological importance. Also, evidence is mounting that exogenous insults acting on fetus can cause irreversible damages to chromatin (called epimutations), impairing gene functions in the ensuing adults and even their offspring, which has profound medical implications, as many human diseases including asthma seem to have fetal origins. However, the mechanisms underlying these fascinating epigenetic phenomena are poorly understood. We are using the well-defined CD4 gene as a model to address this issue. A notable feature of CD4 regulation is that CD4 is reversibly repressed in early T cells, but irreversibly repressed (or [unreadable]epigenetically silenced[unreadable]) in mature CD8 cells. The two forms of repression involve distinct chromatin structures at the CD4 locus, but the relationship between the two structures is unknown. Neither is it clear how exactly the chromatin in CD8 cells mediates epigenetic silencing. To attack these problems, we have created a novel animal model where the CD4 locus can be conditionally manipulated at the genetic and epigenetic levels. Our preliminary data suggest that reversible repression and epigenetic silencing are interconvertible (Aim 1), and that the chromatin in CD8 cells is endowed with an epigenetic memory that acts to restore CD4 repression after forced disruption (Aim 2). We will explore the mechanisms underlying these phenomena by examining the chromatin structure of the CD4 locus. In addition, we are able to induce an epimutation at the CD4 chromatin that permanently changes CD4 regulation. We will define the molecular basis of the epimutation and how it arises and propagates (Aim 3A);our hypothesis is that epimutations are based on a network of self-reinforcing chromatin modifications, which we will seek to identify. Furthermore, we will seek to repair the epimutation (Aim 3B) and to use ES cells to perform in-depth mechanistic study of the epimutation (Aim 3C). Our work will set up a new paradigm in epigenetic research, and will solve several vexing problems regarding the mechanisms of CD4 regulation.